Light-detecting devices such as photodiodes are useful for optical signal reception in optoelectronic networks. Optical signal reception may include reception of multiple channels (or wavelengths) of light (i.e., a multi-channel signal). One approach to receiving a multi-channel signal is the use of multiple cascaded detectors configured to detect light of different wavelengths.
A cascaded detector array in a receiver optical sub-assembly (ROSA) equipped with multiple lenses for focusing the light on individual detectors may receive one or more signals (e.g., light beams) having a focal point that is not perfectly on axis, or in alignment, with a center line of the light beam(s). Also, in such multi-channel systems, the focal points of different signals or light beams may not be equally spaced apart.
Newer generations of multi-channel optical receivers may require four or more detectors mounted within a small area of the ROSA package or substrate. However, small packaging typically does not allow for typical free space alignment. Also, small transceiver (e.g., ROSA) packaging may have narrow wavelength filtering requirements. Furthermore, in a cascaded wavelength drop design (e.g., where successive channels have center wavelengths that differ in a particular direction and/or by a predetermined amount), there is a need for an extra degree of freedom to allow for the use of individual filters with the lens array.
For example, referring to FIG. 1, a multi-channel receiver optical sub-assembly (ROSA) 100 can use passive elements with collimated beams 141, 143, 145 and 147 that emerge from a multiplexer/demultiplexer 120 and integrated lenses 132, 134, 136 and 138. This design is intended to achieve exact X-Y focal points 142, 144, 146 and 148 so that a detector array (not shown) can be placed at the focal points. In FIG. 1, this is represented by equal distances X between adjacent focal points 142 and 144, 144 and 146, and 146 and 148 along the X axis (i.e., the line parallel to and above a line bisecting the four lenses 132-138), with zero offset along the Y axis. A detector array can be placed on a substrate such as a printed circuit board with high precision die attach equipment. Then, the detector array and/or a fiber to be inserted into opening 113 in ROSA substrate/package 110 can be aligned to achieve maximum responsivity or sensitivity in the detectors. Unfortunately, any beam misalignment from the intended X-Y target 142, 144, 146 or 148 of one detector relative to the other detectors cannot be corrected with this method of alignment.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.